1. Field of the Invention
The present invention relates to a method for moulding a fusible core from molten metal, in which the molten metal is forced back under pressure into a suitable mould.
2. Background of The Related Art
The fusible cores used for moulding hollow plastic articles having portions with re-entrant angles must be free from:
casting gouges, such gouges generally being produced by sudden variations in filling cross-sections or by insufficient filling speeds caused, in particular, by back pressures in the mould which are difficult to control. PA1 burrs, despite producing mould joint faces which are as perfect as possible. However, such faces result in an increase in the back pressure which involves the need for the provision of degassing vents, the latter not exhibiting a behaviour which is constant and reliable over time. PA1 surface holes, covered with a thin metal film, which, due to the collapse of this film during subsequent use of the fusible core in injection moulding, give rise to surface defects in the injection-moulded plastic articles. PA1 the back pressure in the mould varies over time particularly according to the condition of the degassing vents which tend to block up and, that being the case, even by imposing a pressure profile during moulding, it is impossible, during a series of moulding cycles, to know the filling level of the mould at all times, particularly when the mould has variable cross-sections. PA1 the metal is forced back in an oscillating manner and often contains gas bubbles, which gives rise to the production of gouges and the formation of holes in the fusible cores produced in this manner.
A technique which is widely used for moulding fusible cores consists in forcing back the molten metal, in a duct connected to the mould, through the effect of a gaseous pressure.
This technique, however, has two serious defects:
Moreover, during moulding of certain types of fusible core, it is important to be able to tilt the mould during filling thereof in order to prevent any overflow of metal and hence the formation of waves.
In this case, use of the technique described above must be excluded, as it does not make it possible to know the level of the metal front in the mould at all times, and it becomes impossible to determine the precise moment when it is required to tilt the mould.
Another possible technique consists in using a positive displacement pump which permits a non-ambiguous relationship between the level reached by the molten-metal front in the mould and the amount of molten metal delivered by the pump.
However, this technique does not, a priori, permit uniform filling of moulds, in particular in the case of moulds having variable cross-sections and/or requiring one or more tilting operations during filling.
It is also known (Giesserei 1991, 78, No. 5, 4 March, p. 146-150) to inject the molten metal into the forming mould by means, for example, of a piston pump and, in this case, to measure the pressure of the fluid driving the pump and to slave this pressure to a predetermined profile, for example by means of a regulation loop.
However, in such methods, no account is taken of parasitic increases in pressure which are due, for example, to frictions which may be very considerable and even predominant given the low viscosity of the molten metal. This results in the measured and controlled pressure not reflecting the degree of filling of the mould and, consequently, in these methods not permitting efficient control of the filling of the mould and hence the production of fusible cores which are free from defects.
Consequently, the present invention relates to a method for moulding fusible cores using a positive displacement pump which, in a simple manner, permits uniform production of fusible cores which are virtually free from defects.